Biologically active compounds such as proteins, enzymes, and nucleic acids have been delivered to the cells using amphipathic compounds that contain both hydrophobic and hydrophilic domains. Typically these amphipathic compounds are organized into vesicular structures such as liposomes, micellar, or inverse micellar structures. Liposomes can contain an aqueous volume that is entirely enclosed by a membrane composed of lipid molecules (usually phospholipids) (R. C. New, p. 1, chapter 1, “Introductiont” in Liposomes: A Practical Approach, ed. R. C. New IRL Press at Oxford University Press, Oxford, 1990). Micelles and inverse micelles are microscopic vesicles that contain amphipathic molecules but do not contain an aqueous volume that is entirely enclosed by a membrane. In micelles the hydrophilic part of the amphipathic compound is on the outside (on the surface of the vesicle) whereas in inverse micelles the hydrophobic part of the amphipathic compound is on the outside. The inverse micelles thus contain a polar core that can solubilize both water and macromolecules within the inverse micelle. As the volume of the core aqueous pool increases the aqueous enviromnent begins to match the physical and chemical characteristics of bulk water. The resulting inverse micelle can be referred to as a microemulsion of water in oil (Schelly, Z. A. Current Opinion in Colloid and Interface Science, 37–41, 1997; Castro, M. J. M., Cabral, J. M. S. Biotech. Adv. 6, 151–167, 1988).
Microemulsions are isotropic, thermodynamically stable solutions in which substantial amounts of two immiscible liquids (water and oil) are brought into a single phase due to a surfactant or mixture of surfactants. The spontaneously formed colloidal particles are globular droplets of the minor solvent, surrounded by a monolayer of surfactant molecules. The spontaneous curvature, H0 of the surfactant monolayer at the oil/water interface dictates the phase behavior and microstructure of the vesicle. Hydrophilic surfactants produce oil in water (O/W) microemulsions (H0>0), whereas lipophilic surfactants produce water in oil (W/O) microemulsions. When the hydrophile-lipophile properties of the surfactant monolayer at the water/oil interface are balanced bicontinuous-type microemulsions are formed (H0=0).
Positively-charged, neutral, and negatively-charged liposomes have been used to deliver nucleic acids to cells. For example, plasmid DNA expression in the liver has been achieved via liposomes delivered by tail vein or intraportal routes. Positively-charged micelles have also been used to package nucleic acids into complexes for the delivery of the nucleic acid to cells. Negatively-charged micelles have been used to condense DNA, however they have not been used for the delivery of nucleic acids to cells (Imre, V. E., Luisi, P. L. Biochemical and Biophysical Research Communications, 107, 538–545, 1982). This is because the previous efforts relied upon the positive-charge of the micelles to provide a cross-bridge between the polyanionic nucleic acids and the polyanionic surfaces of the cells. Micelles that are not positively-charged, or that do not form a positively charged complex cannot perform this function. For example, a recent report demonstrated the use of a cationic detergent to compact DNA, resulting in the formation of a stable, negatively-charged particle (Blessing, T., Remy, J. S., Behr, J. P. Proc. Natl. Acad. Sci. USA, 95, 1427–1431, 1998). A cationic detergent containing a free thiol was utilized which allowed for an oxidative dimerization of the surfactant to the disulfide in the presence of DNA. However, as expected, the negatively-charged complex was not effective for transfection. Reverse (water in oil) micelles has also been used to make cell-like compartments for molecular evolution of nucleic acids (Tawfik, D. S. and Griffiths, A. D. Nature Biotechnology 16:652, 1998). Cleavable micellar systems was not used in this system.
In addition, Wolff et al. have developed a method for the preparation of DNA/amphipathic complexes including micelles in which at least one amphipathic compound layer that surrounds a non-aqueous core that contains a polyion such as a nucleic acid (Wolff, J., Budker, V., and Gurevich, V. U.S. Pat. No. 5,635,487).
Cleavable Micelles
A new area in micelle technology involves the use of cleavable surfactants to form the micelle. Surfactants containing an acetal linkage, azo-containing surfactants, elimination of an ammonium salt, quaternary hydrazonium surfactants, 2-alkoxy-N,N-dimethylamine N-oxides, and ester containing surfactants such as ester containing quaternary ammonium compounds and esters containing a sugar have been developed.
These cleavable surfactants within micelles are designed to decompose on exposure to strong acid, ultraviolet light, alkali, and heat. These conditions are very harsh and are not compatible with retention of biologic activity of biologic compounds such as proteins or nucleic acids. Thus, biologically active compounds have not been purified using reverse micelles containing cleavable surfactants.
Complexation of Nucleic Acids with Polycations
Polymers are used for drug delivery for a variety of therapeutic purposes. Polymers have also been used for the delivery of nucleic acids (polynucleotides and oligonucleotides) to cells for therapeutic purposes that have been termed gene therapy or anti-sense therapy. One of the several methods of nucleic acid delivery to the cells is the use of DNA-polycation complexes. It was shown that cationic proteins like histones and protamines or synthetic polymers like polylysine, polyarginine, polyornithine, DEAE dextran, polybrene, and polyethylenimine were effective intracellular delivery agents while small polycations like spermine were ineffective. As a result the main mechanism of DNA translocation to the intracellular space might be non-specific adsorptive endocytosis which may be more effective then liquid endocytosis or receptor-mediated endocytosis. Furthermore, polycations are a very convenient linker for attaching specific receptors to DNA and as result, DNA-polycation complexes can be targeted to specific cell types.
There are a variety of molecules (gene transfer enhancing signals) that can be covalently attached to the gene in order to enable or enhance its cellular transport. These include signals that enhance cellular binding to receptors, cytoplasmic transport to the nucleus and nuclear entry or release from endosomes or other intracellular vesicles.
Nuclear localizing signals enhance the entry of the gene into the nucleus or directs the gene into the proximity of the nucleus. Such nuclear transport signals can be a protein or a peptide such as the SV40 large T ag NLS or the nucleoplasmin NLS.
Other molecules include ligands that bind to cellular receptors on the membrane surface increasing contact of the gene with the cell. These can include targeting group such as agents that target to the asialoglycoprotein receptor by using asiologlycoproteins or galactose residues. Other proteins such as insulin, EGF, or transferrin can be used for targeting. Peptides that include the RGD sequence can be used to target many cells. Chemical groups that react with sulfhydryl or disulfide groups on cells can also be used to target many types of cells. Folate and other vitamins can also be used for targeting. Other targeting groups include molecules that interact with membranes such as fatty acids, cholesterol, dansyl compounds, and amphotericin derivatives.
Size of a DNA complex may be a factor for gene delivery in vivo. Many times, the size of DNA that is of interest is large, and one method of delivery utilizes compaction techniques. The DNA complex needs to cross the endothelial barrier and reach the parenchymal cells of interest. The largest endothelia fenestrae (holes in the endothelial barrier) occur in the liver and have average diameter of 100 nm. The trans-epithelial pores in other organs are much smaller, for example, muscle endothelium can be described as a structure which has a large number of small pores with a radius of 4 nm, and a very low number of large pores with a radius of 20–30 nm.(Rippe, B. Physiological Rev, 1994). The size of the DNA complex is also important for the cellular uptake process. After binding to the target cells the DNA complex should be taken up by endocytosis. Since the endocytic vesicles have a homogenous internal diameter of about 100 nm in hepatocytes, and are of similar size in other cell types, the DNA is compacted to be smaller than 100 nm.
Compaction of DNA
There are two major approaches for compacting DNA:
1. Multivalent cations with a charge of three or higher have been shown to condense DNA. These include spermidine, spermine, Co(NH3)63+,Fe3+, and natural or synthetic polymers such as histone H1, protamine, polylysine, and polyethylenimine. One analysis has shown DNA condensation to be favored when 90% or more of the charges along the sugar-phosphate backbone are neutralized (Wilson, R. W., Bloomfield, V. A. Biochemistry 18, 2192–2196, 1979).
2. Polymers (neutral or anionic) which can increase repulsion between DNA and its surroundings have been shown to compact DNA. Most significantly, spontaneous DNA self-assembly and aggregation process have been shown to result from the confinement of large amounts of DNA, due to excluded volume effect (Strzelecka, T. E., Rill, R. L. Biopolymers 30, 803–14, 1990; Strzelecka, T. E., Rill, R. L. Biopolymers 30, 57–71, 1990). Since self-assembly is associated with locally or macroscopically crowded DNA solutions, it is expected, that DNA insertion into small water cavities with a size comparable to the DNA will tend to form mono or oligomolecular compact structures.
Micelles and Reverse Micelles
Reverse micelles (water in oil microemulsions) are widely used as a host for biomolecules. Examples exist of both recovery of extracellular proteins from a culture broth and recovery of intracellular proteins. Although widely used, recovery of the biomolecules is difficult due to the stability of the formed micelle and due to incomplete recovery during the extraction process. Similarly, purification of DNA or other biomolecules from endotoxin and plasma is difficult to accomplish. One common method employing Triton results in incomplete separation of the DNA or biomolecules from the emulsion.
Reverse micelles have been widely used as a host for enzymatic reactions to take place. In many examples, enzymatic activity has been shown to increase with micelles, and has allowed enzymatic reactions to be conducted on water insoluble substrates. Additionally, enzymatic activity of whole cells entrapped in reverse micelles has been investigated (Gajjar, L., Singh, A., Dubey, R. S., Srivastava, R. C. Applied Biochemistry and Biotechnology, 66, 159–172, 1997). The cationic surfactant cetyl pyridinuim chloride was utilized to entrap Baker's yeast and Brewer's yeast inside a reverse micelle.
Micelles have also been used as a reaction media. For example, a micelle has been used to study the kinetic and synthetic applications of the dehydrobromination of 2-(p-nitrophenyl) ethyl bromide. Additionally, micelles have found use as an emulsifier for emulsion polymerizations.
Micelles have been utilized for drug delivery. For example, an AB block copolymer has been investigated for the micellar delivery of hydrophobic drugs. Transport and metabolism of thymidine analogues has been investigated via intestinal absorption utilizing a micellar solution of sodium glycocholate. Additionally, several examples of micelle use in transdermal applications have appeared. For example, sucrose laurate has been utilized for topical preparations of cyclosporin A.